ES2962547T3 - System and procedure to evaluate grip safety - Google Patents

Abstract

A system for evaluating grip security, the system comprising: a contact surface having at least a first contact surface region and a second contact surface region, the first contact surface region being configured to resist less slipping than the second contact surface region, a sensor to detect slip in the first contact region. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

System and procedure to evaluate grip safety

TECHNICAL FIELD

The present disclosure relates to a system and procedure for evaluating grip safety.

BACKGROUND OF THE TECHNIQUE

Grasping and lifting objects with robotic grippers is a difficult task. Robotic grippers do not have the ability of the human hand to detect valuable information about the object and the contact interface. In most cases, tweezers do not have tactile feedback. Those that do have tactile feedback usually measure a characteristic, for example grip forces, or detect the object that slips from the grip. Although the design of robotic and prosthetic grippers continues to evolve, trying to emulate the dexterity of the human hand, it is still far from achieving comparable performance.

The field of tactile sensing is very active and aims to fill this gap; However, most existing touch sensors focus on determining normal and tangential forces on the interface. Although these magnitudes are important, there are undoubtedly other tactile parameters that are also important for dexterous manipulation. Two of these parameters are the coefficient of static friction (ms) and the occurrence and extent of incipient slip.

The coefficient of static friction of the contact interface helps determine the minimum gripping force (normal) necessary to hold an object of a given weight (tangential force). In certain gripping postures, if the coefficient of static friction is accurately estimated and the tangential force can be measured, the (normal) grip force can be adjusted to securely hold the object.

Various tactile sensors have been described in the literature to measure the coefficient of static friction and the occurrence and extent of incipient slip, such as that described in "Slip classification for dynamic tactile array sensors" by Barrett Heyneman and Mark R. Cutkosky, in The International Journal of Robotics Research 2016, Vol. 35(4) 404-421. However, many of these sensors suffer from one or more of the following limitations:

(i) the need to explore the object before attempting to grasp it,

(ii) the appearance of gross slip during handling before obtaining a measurement of the coefficient of static friction,

(iii) the inability to provide a continuous measurement of the coefficient of static friction in the event of changing friction conditions, and

(iv) the need to continually control normal and tangential forces.

Some of these problems may be resolved or mitigated or at least an alternative may be provided within the present disclosure.

An alternative to measuring the coefficient of static friction is to detect incipient slippage and adjust the grip force if it occurs. Incipient slip is defined as a relative displacement that occurs in a localized region of the contact interface, while total slip implies a relative displacement across the entire contact interface. However, at present, there is still no dominant and well-established technology to artificially detect slippage, despite the multitude of slip sensors reported in the literature.

Therefore, it would be advantageous to the art if a device were capable of accurately detecting incipient slippage while the grip is still secured, thus allowing modulation of force before complete loss of grip is experienced.

MIT researchers have developed a device to detect incipient landslides called GelSight. It uses a transparent silicone and a camera to measure sliding in the contact area by following the movement of a dot pattern tattooed on the silicone. However, its ability to detect incipient landslide is limited since it uses a flat, contiguous surface. The flat surface limits the establishment of a pressure differential, and therefore differential traction, across the contact interface. The contiguous nature of the sensing elastomeric material discourages independence of motion between different localized regions of the sensing interface, further discouraging the occurrence of incipient slip. The GelSight sensor is also limited to detecting relatively low-frequency touch events, as it relies on image processing of a video stream to detect movement of the silicone at the contact interface.

It would also be advantageous for the existing technique to have a tactile sensor device capable of reliably detecting and signaling an imminent slip, simultaneously estimating the friction, regardless of the material with which it was in contact.

SUMMARY

A system for evaluating grip security is disclosed, the system comprises a contact surface comprising a plurality of protuberances extending from a base surface, wherein the contact surface has at least a first contact surface region and a second contact surface region, the first contact surface region being configured to resist slippage less than the second contact surface region. The protuberances of the first region of the contact surface extend away from the base surface at a shorter distance than the protuberances of the second region of the contact surface. The disclosed system further comprises a sensor for detecting slippage in the first contact region.

Slip detection can be used to evaluate grip safety and provide feedback within the system so that grip force is increased to increase grip safety.

In some preferred embodiments, the protrusions are deformable.

In some forms, the bumps are compressible. In some forms, the protuberances are shaped like elongated pillars.

In some embodiments, the protuberances may be positioned as an assembly.

Also disclosed is a method for evaluating grip security, the method comprising detecting slippage on a contact surface, wherein the contact surface comprises a plurality of protuberances extending from a base surface, wherein the contact surface has at least a first contact surface region and a second contact surface region, the first contact surface region being configured to resist sliding less than the second contact surface region. The protuberances of the first region of the contact surface extend away from the base surface at a shorter distance than the protuberances of the second region of the contact surface.

The method comprises the step of detecting the slip in the first region of the contact surface by means of a sensor.

Varying the height or distance of the protuberance with respect to the base surface is advantageous, since it allows the system to detect incipient slippage.

The difference in heights of the protuberances causes them to experience a normal force dependent on the distance the protuberance extends from the base surface when the protuberances are compressed to the same final height. When a tangential force is also applied, assuming that the protrusions do not bend appreciably, all protrusions that have the same cross section experience the same tangential force. Consequently, the relationship between the tangential force and the normal force experienced by each protuberance varies with the difference in height of the protuberances. If it is further assumed that the sensor surface maintains a constant coefficient of static friction, as the tangential force increases, the protrusion subjected to the lowest normal force (i.e., the shortest protrusion when the device is unloaded) will slide first when the ratio between the tangential force and the normal force is greater than the coefficient of static friction. As the tangential force continues to increase, the next shorter bump will slide, and so on, until the tallest bump has slid. Thus, each incipient sliding event acts as a warning that grip/normal force must be increased to maintain the grip of a stable object.

In another embodiment of the disclosure, the movement of the individual protuberances may be independent of each other. The independent movement of at least two pillars is advantageous because it allows measurement of relative movement at the contact surface, which can occur only at different levels along the contact surface.

In another embodiment of the disclosure, the normal force experienced by a protrusion in the first contact region is less than that of a protrusion in the second contact region.

In another embodiment of the disclosure, the protuberances are pillars.

In another embodiment of the disclosure, a first end of a protrusion is connected to the base surface, and an opposite second end of the protrusion forms a rounded or spherical or non-flat tip.

In another embodiment of the disclosure, the contact surface is primarily made of silicone.

In some embodiments of the disclosure, the base surface may be planar, while in other embodiments the base surface may be non-planar. When the surface of the grasped object is flat, the relative compression of each protrusion can be easily determined with respect to a common base surface, even if the base surface is not flat. Forces in three dimensions can be measured regardless of the shape of the surface or whether the base surface is not flat.

The sensor can have various shapes.

In some forms, a sensor is placed behind the base surface at a location adjacent to each cavity.

In some forms, the sensor is adapted to detect incipient slip.

In some forms, the sensor is adapted to estimate friction.

In some forms, between the sensor and the cavity there is an opening that is smaller in diameter than the cavity.

In some forms, the sensor comprises a quadrant photodiode configured to detect light emanating from LEDs located on one side of the base surface cavity within a given protrusion, which is reflected from a reflector located at the distal end of the cavity or near it and travels through the opening to the sensor.

In some forms, the protuberances have an internal cavity, and the sensor comprises a CCD array, the system being configured such that the CCD array detects light emanating from the base surface into the cavity, reflecting from a reflector located in the distal end of the cavity, and traveling through an opening in the base surface towards the sensor.

In some forms, compression of the protrusions in the z-axis normal to the base surface results in expansion of the diameter of the detected light spot.

In some forms, the protrusions have an internal cavity, and the sensor comprises a light-sensitive CMOS array, the system being configured such that the light-sensitive CMOS array detects light emanating from the base surface toward the cavity, reflecting from a reflector located at the distal end of the cavity, and traveling through an opening in the base surface towards the sensor.

In some embodiments of the disclosure, the speed at which incipient slip warnings are signaled by the sensor, the speed at which slip events are detected, or a number of warnings, may be used to indicate a level of urgency. with which corrective action is required, or the magnitude of the force necessary for corrective action.

In some forms, the method comprises measuring the displacement of the protuberances in three spatial dimensions, estimating the forces applied to the tip in three spatial dimensions.

In some forms, the system detects slippage when one or more protrusions no longer move at the same speed as other protrusions in the assembly. In some forms, the system detects slippage through vibrations. In some forms, upon detection of slip, the system reviews the relationship between the tangential and normal force at the time of slip to allow an estimate of the friction coefficient.

In some forms, the method comprises measuring the displacement of the tip of the protrusions in three dimensions with a high resolution, such that the vibration of the tip is measured, using the measurement of the vibration to estimate the texture or detect slippage. .

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with reference to the accompanying drawings in which

Figure 1A is a cross-sectional view of one embodiment of a contact surface of the disclosure; Figure 1 is a cross-sectional view of the embodiment of Figure 1 in a compressed state;

Figure 1C is a cross-sectional view of the compressed embodiment of Figure 1;

Figure 2 is a perspective view of a contact surface of the disclosure;

Figure 3 is a perspective cross-sectional view of a contact surface of the disclosure;

Figure 4 is a cross-sectional view of an embodiment of the contact surface integrated with a sensor system according to the disclosure;

Figure 5 shows the manufacture of a prototype of an embodiment of the disclosure, showing both the mold and the resulting silicone profile;

Figure 6 shows a testbed for testing at least one embodiment of the disclosure;

Figure 7 shows a single frame of video captured during the testing procedure;

Figures 8A - 8C provide a graphical view of the tangential to normal force ratio at which each pillar slides at each normal force level. Figure 8A shows the high friction surface, Figure 8B shows the basic friction surface and Figure 8C shows the low friction surface;

Figures 9A - 9C provide a graphical representation of displacement and force detection using one embodiment of the disclosure. Figure 9A provides a reference displacement of the abutment tip, Figure 9C provides a reference force data, and Figure 9B shows the data output of the optical sensor;

Figure 10 shows the detection of vibration response using one embodiment of the disclosure.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form part of the detailed description. The illustrative embodiments described in the detailed description, represented in the drawings and defined in the claims, are not intended to be limiting.

In Figure 1, a system for evaluating grip security is shown, comprising a contact surface 10 having at least a first contact surface region 12 and a second contact surface region 14. As illustrated, The contact surface is in the form of a plurality of protuberances 16 extending from a base surface. Only two protuberances 16 are shown in Figure 1, however, it will be apparent to the person skilled in the art that the contact surface may comprise a plurality of protuberances 16 extending from the base surface 18. The protuberances 16 are shaped like elongated pillars with an attachment end 19 engaged or integrated into the base surface 18 and extending to a tip 20 having, in some embodiments, a hemispherical end profile. In the illustrated form, the elongated pillars have similar cross-sectional dimensions and different extension lengths from the base surface. In this illustrated form, the variation in the distance that the protrusion extends from the base surface defines the first contact surface region 12 and the second contact surface region 14. Specifically, the first contact surface region is configured to resist sliding less than the second contact surface region. In the illustrated form, only two elongated protuberances are shown, which have different heights from each other.

Figure 1 is a simplified model of a possible embodiment, in which a longer central protuberance or pillar, of height 1, is surrounded by eight shorter outer pillars, each of them of height 1. Other embodiment variants include several pillars of different heights.

Figure 1A illustrates two protuberances 16 when not compressed, each of which is pillar-shaped and of equal diameters, D, but different heights, read it.

In Figure 1B, the device 10 is illustrated in contact with a flat surface 24, such as the surface of an object being grasped. In this figure, both pillars are compressed by a gross normal force Fn to the same final height, resulting in a different normal (compression) force in each pillar.

Figure 1C illustrates the addition of a tangential force that acts to shear the flat surface 24 with which the sensor is in contact, whereby each of the protuberances 16 also experiences a tangential force. When no protuberance slides against the surface, and assuming that the pillars cannot bend appreciably, they all experience the same tangential force, and the sum of the tangential forces is equal to the gross tangential force, Ft.

In some embodiments of the disclosure, the movement of the individual protuberances 16 may be independent of each other. The independent or partially independent movement of at least two pillar-like protuberances 16 is advantageous because it allows measurement of relative movement at the contact surface, which can occur only at different levels across the contact surface 10.

In some embodiments of the disclosure, the contact surface 10 is primarily made of silicone.

As shown in Figure 1C, the protrusions 16 of the first contact surface 12 are subjected to a lower normal force than the longer protrusion 16' of the second contact surface 14. In some forms, the coefficient of Static friction is the same for each bump; The outer protrusions of the first contact surface region 12 of the embodiment shown in Figure 2 will slide with a lesser gross tangential force than the central protrusion 16' of the second contact surface region.

If the spring constant (k) and the diameter (D) of a single protuberance are known, and the tangential and normal force at the moment of sliding of the shorter protuberances are measured, it is possible to predict the relationship between the gross tangential force and the normal force at which the longest protrusion will slide. The static friction coefficient is therefore an estimate of the relationship between the tangential force and the normal force at the moment of sliding for the longest protuberance. In some forms, the brute force through the device is measured and distributed among the protuberances, assuming that the material is linear elastic. In some forms not illustrated, force may be detected individually for protrusions or groups of protrusions.

Ideally, since the bump does not slide, it should deflect at the same speed as the surface it is compressed against. On the contrary, when the protrusion slides, the deflection velocity should tend towards 0 mm/s. In practice, due to the bending of the protrusion or pillar, the sliding moment was determined as the moment when the deflection speed of a shorter protrusion decreases to 20% of the deflection speed of the longer protrusion, conditional on the deflection speed of the longest protuberance being sufficiently large.

With more protrusions and more height differences, a wider range of frictions and normal forces can be taken into account, and more warnings are possible to avoid loss of the object as the tangential force increases. The rate at which warnings are issued, as well as the number of warnings, could indicate the urgency with which corrective action is required. In addition, with each notice you can learn more information about the contact interface.

Both with and without continuous monitoring of normal and tangential force, the disclosed device can be used advantageously to improve dexterous manipulation in robotic and prosthetic grippers. Without ongoing monitoring, the rate at which warnings are issued, as well as the number of warnings, could continue to indicate the urgency with which corrective action is required, as well as the magnitude of the action. With continuous force monitoring, it may be possible to also determine the coefficient of static friction, and grip corrections would be better informed. Regardless of the type of monitoring, a warning may be issued when incipient slippage is detected.

In some embodiments of the device, the contact surface 12 is planar, so that the relative compression of each protuberance can be determined.

The system in some forms further comprises a sensor, or system of sensors, adapted to measure slip in the first contact region 12 to detect incipient slip. The sensor can have various shapes.

As shown in Figure 2, the contact surface comprises a base surface 18 with a plurality of protuberances extending therefrom. In some forms, the contact surface further comprises a lower bearing surface 41 and an upper bearing surface 40. In some forms, the upper bearing surface includes a plurality of openings through which the protuberances may extend. In some forms, the bottom surface 41 may include support or cavities for supporting sensors or other parts of the system.

In some embodiments of the device, such as those illustrated in Figures 3 and 4, an illuminated reflector 31, an aperture 32 and a light sensor 33 form a pinhole camera configuration 30, which allows measurement of the three-dimensional deviation of the illuminated reflector 31. This deviation correlates with the three-dimensional deviation of the tip of the protuberance 16. A pinhole camera configuration 30 is one in which light from a source 34 passing through an aperture 32 projects an inverted image of the source onto a screen or sensor 33 located below. The light source 34 is projected into a cavity 35 within each protrusion 16 to be reflected by a reflector 31 at the distal end of the cavity 35, and back through the pinhole opening 32 to a sensor 33 mounted at a distance from the pinhole opening 32, behind the base surface 18.

In some embodiments of the device, the light source 34 originates from a small circular disk, onto which a circular spot of light will be projected. By monitoring the position of the light spot, the sensor 33 can detect the three-dimensional position of the source 34 with respect to the aperture 32. In some embodiments, the light spot enlarges when the protrusion is compressed, which means that the deformation can be measured. along an axis normal to the base.

The device also allows measuring the change or deformation in three dimensions, that is, in the x and y axes of a plane tangent to the base surface and in the z axis that extends normal to the base surface at the point where a given protuberance extends from the surface.

Visual representation of the change or deformation in all three axes allows measurement of the three-dimensional displacement applied to the bump or tip of the bump. This three-dimensional visual representation of displacement comprises visual representations of the motion of the light spot to demonstrate angular motion of the protuberance in the x-y plane tangential to the base surface and visual representation of motion in the z axis normal to the base surface using a change in the size of the light spot. This visual representation allows the forces applied to the protuberance to be determined in all three dimensions. This three-dimensional force measurement also allows estimation of friction when sliding occurs.

In some forms, the system detects slippage when one or more protrusions no longer move at the same speed as other protrusions in the assembly. In some forms, the system detects slippage through vibrations. In some forms, upon detection of slip, the system reviews the relationship between the tangential and normal force at the time of slip to allow an estimate of the friction coefficient.

Additionally, measuring forces in three dimensions allows the bump assembly to estimate torque, as the x-y deflection or curvature of the force around the z-axis can be detected. This torque estimation allows for increased grip security, as the grip force can be increased to take into account the increase in torque as appropriate.

Measurement of three-dimensional displacement with a large bandwidth and very high spatial resolution allows vibration in protrusions to be detected. Vibration detection provides an alert related to slip events. Alternatively, vibration detection provides a means of detecting texture and the ability to distinguish between textures. Additionally, detection of vibration can lead to transduction of voice or music or other sound from a vibrating surface.

In some embodiments of the device, a cavity 35 within the protuberance may contain a reflecting disk 31 illuminated by the LEDs 34 and the opening 32.

In some embodiments of the device, the cavity 35 is conical in shape.

In some embodiments of the device, a sensor 33 in the form of a quadrant photodiode that is located below the aperture 32 can detect the position and/or size of the projected light spot retransmitted from the reflector 31. In such an embodiment, the calculation of the position of the luminous point correlates with the position of the tip of the protuberance in the x-y plane tangential to the base surface. This calculation uses a relatively simple formula whereby each axis is calculated by subtracting the difference between the sensor halves and normalizing to the total light received. Similarly, the size of the bright spot correlates with the position of the tip of the protrusion along the z-axis normal to the base surface which correlates with compression or release along the z-axis normal to the surface. base. The calculation of the tip position along the z-axis can be calculated by simply measuring the intensity of the light incident on the photodiodes. The simplicity of these calculations makes the design suitable for a microprocessor, even with large sensor arrays.

In some embodiments of the device, a pinhole camera configuration 30 is capable of measuring the direction and magnitude of the deflection of the protrusion 16 in two dimensions by examining the relative proportions of light illuminating each of the four quadrants of the photodiode sensor 33; As the protrusion 16 is deflected the direction of the light beam shining through the pinhole aperture 32 will change. For smaller scale embodiments of the device, the photodiode sensor 33 could be replaced with a CCD or CMOS light sensitive array. Variations and modifications may be made to the parts described above without departing from the spirit or scope of the disclosure.

Examples

In some illustrated embodiments, the estimation of the forces on the protuberances or pillars may be carried out as follows.

The material is assumed to behave as a linear elastic according to Hooke's Law. It is observed that the brute normal force is the sum of the normal forces acting on each pillar.

In the case of a single central pillar, surrounded by eight outer pillars, this means:

Fnc — kAlc,

( the )

Fm=kAl0= /c(A¿c - d) =Fnc - kd, and (Ib )

FN=Fnc+&FN0 = 9kAlc- 8kd,( l e )

where is the spring constant of the elastic material of the pillars. Therefore:

P_Fn kd

h N O - — -9(2b)

When a tangential force is also applied to the sensor by shearing the surface it is in contact with, each of the pillars also experiences a tangential force (see Figure 1C). When all the pillars are glued to the surface (not sliding), and assuming that (i) the compressive stress on the pillars is small in relation to their height, (ii) the difference in height between the longest pillars and the shorter is also small relative to the height, and (iii) the pillars do not bend appreciably relative to their height, then they all experience approximately the same tangential force, and the sum of these tangential forces is equal to the gross tangential force. ,Ft.In the case of a single central pillar, surrounded by eight outer pillars, this means:

where Ft is the tangential force on the central pillar and F is the tangential force on one of the outer pillars.

Since the outer pillars are subjected to a smaller normal force, and j s is the same for each pillar, the outer pillars will slide with a smaller gross tangential force than the central pillar. The outer pillars will begin to slide when:

F to<> í h Fno ■>(4)

This occurs when the gross tangential force is:

While the outer pillars slide and the center pillar remains stuck, the outer pillars contribute a limited amount of tangential force to the gross tangential force, due to the coefficient of kinetic friction (j*):

F t o = ^ x F n o i

(6a)

and the central pillar will begin to slide when:

Ftc>PsFnc-(6b)

This occurs when the gross tangential force is:

Now, js is always greater than or equal to j*, however, if it is assumed that j s = j*, then Eq. (7) can be simplified to:

Combining equations (5) and (8) we obtain:

that is, if k and d are known, and the gross tangential and normal force at the moment of slip lospsO psC are measured

outer pillars (TandNrespectively), it is possible to predict the relationship between the gross tangential force and the normal force at which the central pillar will slide; That is, it is possible to estimate the coefficient of static friction by detecting when the outer pillars are sliding and then examining the forces at that time.

In some forms, the diameter of the sensor base (from which the pillars emanate) Dtotal = 80 mm with a thickness of 3 mm, and each of the cylindrical pillars had a diameter of D = 10 mm and a hemispherical termination, with a separation between centers of approximately 15 mm. Hemispherical ends were chosen as flat ends with sharp edges would cause large compressive forces at the edge of the pillar contact zone before it slipped. In this version of the sensor (as in the previous simple model), eight outer pillars surround a single central pillar, in a 3 x 3 reticular arrangement. The height of the central pillar was = 15 mm, and that of the outer pillars = 15 mm. =14mm; That is, the difference in height between the central pillar and the outer pillars is d =1 mm. To manufacture this prototype, silicone was poured into a 3D printed ABS plastic mold.

The mold was printed in thermoplastic with a 3D printer. To smooth out the 3D printing lines on the mold surface, it was suspended over an acetone vapor bath for 3 hours at room temperature. Figures 5A and 5B show the mold before and after the acetone vapor bath, respectively.

As a material for the prototype, a two-component silicone was used, harmless to the skin, with low viscosity to facilitate its fluidity and a short curing period. The two components were mixed in equal parts according to the manufacturer's instructions and pouring was carried out in a single pour. No degassing was necessary, but the silicone was poured from a height that allowed better control of the pouring stream, and the mold was gently shaken to remove any bubbles present in the silicone. The silicone was released from the mold after curing (see Figures 5C-5E). Figure 5C shows a silicone prototype, top view; Figure 5D shows the prototype with mounting bracket; Figure 5E shows the silicone prototype, side view.

To validate the performance of the prototype, several experiments were conducted to apply normal and tangential forces to the prototype. To carry out these test procedures, a test bench was used consisting of an XYZ stage, a 3D force/torque sensor, the prototype, a transparent acrylic surface and a camera. The test bench and test procedures are described below.

Figure 6 shows an exemplary testbed for testing a prototype of at least one embodiment of the disclosure. The figure shows the XYZ stage labeled A, the acrylic surface labeled B, the prototype labeled C, the 3D force/torque sensor labeled D, the video capture platform labeled E, and the support frame labeled as F.

The , then shear the acrylic surface across the surface of the prototype. Each of the stages has a travel of 25 mm with a maximum speed of 50 mm.s-1, and a precision of 0.1 pm with step sizes up to 0.3 pm. The compression produces a normal force that acts on the pillars and the shearing of the surface when it comes into contact with the object to be held produces tangential forces in each of the pillars.

A 3D force/torque sensor (Mini40, SI-80-4, ATI Industrial Automation, Apex, NC, USA) was mounted between the prototype and a support frame (see Figure 6D). The forces and torques acting on the prototype were sampled at 1 kHz with a PowerLab 16/35 data acquisition unit (AD Instruments, Bella Vista, NSW, Australia).

Video of the prototype in contact with the transparent acrylic surface was captured with the native video recording application of an iPhone 6 16 GB (model A1586). The iPhone was placed on a platform so that the pillars were visible through the transparent acrylic surface from (approximately 100 mm) directly below with the central pillar located in the center of the image. The iPhone was connected to a MacBook Pro running QuickTime Player 10.4 to record the iPhone screen in .MOV format with a resolution of 1334 x 750 pixels at 59.97 fps (frames.s-1). Camera calibration was performed using the MATLAb camera calibration application (R2014b, Mathworks, Natick, MA, USA). Lens distortion coefficients (radial and tangential) were calculated and at the edges of the sensor (beyond the maximum deviation of any of the pillars) the distortion was no more than 1.1 pixels, corresponding approximately to 0 .12mm. In comparison, the pillar tracking points measure approximately 5 pixels in diameter. Since this distortion has the effect of biasing the measurement of the deviation of the pillars, the effect on the results consists only of modifying the moment at which it is determined that the pillars have slipped with respect to the platen; however, the determination of this event time depends much more on the slip detection rule used.

A small hole was created with a pin at the center point of the selected pillars, which was filled with black ink to form a reliable marker for tracking during video analysis. Additionally, a black and white checkerboard composed of a row of three 10 mm squares was attached to the acrylic surface to provide a reference point for tracking the position of the surface, as well as to provide a reference for the conversion of spatial units (pixels to mm) which was reasonable due to the negligible lens distortion.

The analyze the sliding behavior of the pillar (see below) at a speed of 2.5 mm.s-1. The XYZ stage is held in this position for 1.5 s and then moved laterally at a speed of 2.5 mm.s-1 for a total of 15 mm. Next, the stage moves vertically away from the prototype to return to the initial height, thus relieving the forces, and moves laterally to return to the initial position.

By recording the position of the XYZ plate that gives rise to the desired normal force (5, 7.5, 10, 12.5 and 15 N), the spring constant can be calculated according to Hooke's law. Since, at these normal force levels, the nine pillars of the sensor are compressed against the acrylic surface, the spring constant k is equal to 1/9 of the slope of the line defined by the position of the platen versus the force normal.

To ensure that the three surfaces used for testing had different values of js and that the js of each surface remained constant throughout the tests, it was necessary to measure the js . The js was measured by performing the above protocol at a normal force of 0.5 N - at this normal force, only the central pillar makes contact with the surface. Friction was measured before and after testing the behavior of the pillar for each combination of friction condition and normal test force (see below).

The behavior of the sensor was tested with five different levels of normal force: 5, 7.5, 10, 12.5 and 15 N. The XYZ stage was programmed to apply the normal and tangential forces as described above. At the same time, force/torque signals from the ATI sensor were recorded and video was captured of the abutments in contact with the acrylic surface.

Three surfaces with different friction properties were used: (i) ethanol-cleaned acrylic (base friction condition), (ii) olive oil coated acrylic (low friction condition), and (iii) thin layer coated acrylic. of soap that has been allowed to dry (high friction condition).

For each combination of normal force (5, 7.5, 10, 12.5 and 15 N) and surface (acrylic: oil coated, alcohol cleaned, soap coated), the following tests were performed: friction test at 0.5 N (once), abutment behavior test at the desired normal force (five times), friction test at 0.5 N (once).

To eliminate any high-frequency noise in the force signals, a 2nd-order low-pass Butterworth filter with a cutoff frequency of 10 Hz was applied.

The recorded videos were used to monitor the deflections of the central pillar and one of the eight outer pillars of the prototype during the lateral movement of the acrylic surface. The Kanade-Lucas-Tomasi algorithm was used to track points in MATLAB (Mathworks, Natick, MA, USA). Three points were followed throughout the video recording: (i) the center of the central pillar, (ii) the center of an outer pillar - chosen as the pillar directly anterior to the central pillar in the direction of shear - and (iii ) a reference grid point (to control the position of the acrylic surface).

The result of point tracking gives the deflection of the center and outer pillars relative to the position of the acrylic surface and subsequently relative to the undeflected position of each pillar. A frame of video tracking and abutment deflection is shown in Figure 4. A 5 Hz 2nd order low-pass Butterworth filter was then applied to remove tracking fluctuations from the deflection data.

It was necessary to synchronize the filtered force/torque and deflection signals since the original data was recorded on two different devices. At the end of each stimulus, the XYZ stage was accelerated in the negative Z direction (normal to the pillars) to retract the acrylic surface away from the sensor, in order to discharge the normal force. The result is a large acceleration of the measured normal force, as well as the calculated deflection of the central pillar, as the tangential bending force is suddenly removed. The large negative peaks in the second derivatives with respect to time of the filtered normal force and deflection of the central pillar were used to synchronize the force and deflection data.

Ideally, since the stage is moving at a speed of 2.5 mm.s'1, if the pillar is stuck (not sliding), it should also be deflecting at a speed of 2.5 mm.s-1 at its tip, and when the pillar slides, the deflection speed should become 0 mm.s-1. However, this does not occur in practice and, due to the bending of the pillar, it first appears to move at the same speed as the platen, although this speed slowly decreases as the pillar appears to roll at the point of contact. Therefore, the sliding moment was determined heuristically as the moment at which the deflection velocity of the pillar (i.e., the first derivative of the deflection position with respect to time) decreases to 5% of the stage velocity. (i.e. when the pillar deflection velocity first drops below 0.125 mm.s-1) - this is close to zero, but above the frame-to-frame noise level of the pillar deflection velocity . In this work to test the working principle of the prototype, this threshold was sufficient to identify the sliding moment, however, in other real situations the detection algorithm will undoubtedly need to be more complex/robust. In some forms, each protrusion can be instrumented internally using the light and pinhole procedure described above to measure its deflection and vibration of the protrusion. In some ways, this may mean that the slip event will be less ambiguous.

The ratio between the tangential force and the normal force at the moment of sliding (determined by video analysis) of the central pillar (the only pillar in contact at 0.5 N of normal force) was taken as the estimate of j s.

The working principle of the sensor is that the outer (shorter) pillar should slide under a smaller tangential force compared to the middle (taller) pillar. To determine if this is true, the tangential and normal forces at the moment of sliding of the outer and central pillars, respectively, were determined for _usC / rsC

Ms -l'T /t'N(Eq. (9))

compare them. A prediction of was also calculated from the tangential forceslpsO

and normal measurements when the outer pillar slides (<, Ft and>Nrespectively) and a comparison was madeusC/ Z7SC

with the measured forces<I F t / F n>being measured some time later, when the central pillar finally slides.

Results

The positions of the platen (mm) for each of the normal forces (N) were used to calculate the spring constant,k,of the sensing pillars. The spring constant k is the gradient of the line of best fit divided by the number of pillars: k = 1.174 N/mm.

It was observed that when the acrylic plate begins to shear (approximately at 2.4 s), a decrease in the normal force occurs. This is to be expected, as the XYZ stage is programmed to remain at the same height while shearing the acrylic surface, and the sensor pillars bend, meaning the effective height of the sensor decreases slightly. The js was taken as the ratio between the tangential force and the normal force at the moment of sliding of the central pillar.

A video frame captured during the testing procedure is shown in Figure 7. The red cross is the original position of the marker; The blue cross is the current position of the marker, which moves with the acrylic plate. The values highlighted in yellow are the deflections (mm) of the marker (top), the center pillar (center), and the outer pillar to the left of the center pillar (bottom).

Referring to Figure 8, a graphical view of the tangential to normal force ratio in which each pillar slides at each normal force level is shown for the A) high, B) base, and C) low friction surfaces. Markers are mean values and error bars extend to ±SD. The dashed horizontal line indicates the ms measured with a normal force of 0.5 N.

As shown in Figures 9 A to 9C, the optical sensor is capable of measuring the displacement of the protrusions and the force on them. In the graphical representations, Figure 9A provides the reference displacement. The XY coordinates are obtained by following a spotted point on the tip of a pillar from above with a video camera. We place a transparent sheet of thick plexiglass in contact with the tip of the pillar using a robotic stage and move it to cause an XYZ displacement. The robotic stage tells us the Z coordinate.

Figure 9C shows the reference force. It is obtained using a commercial 3-axis force sensor.

Figure 9B shows the measurement of displacement and force using four photodiodes after simple preprocessing. If there are four photodiodes arranged in a quadrant pattern as shown,

PQ

R.S.

The perceived light intensity can then be preprocessed as follows to obtain the central graph.

(i.e. sum of all)

(i.e. left minus right, normalized)

(i.e. top minus bottom, normalized)

Finally, two mapping functions are learned to map the values that measure displacement and force shown in Figure 9B to: the values in Figure 9A that show the reference displacement in three dimensions, or the values in Figure 9C that show the reference forces in three dimensions.

As shown in Figure 10, the sensors can detect vibrations. In this example, the tip of the bump was in contact with a shaker vibrating along a single axis. The attached image shows the results of a test in which a vibration of 10 microns (0.01 mm) at 330 Hz was applied in the Z axis of the abutment (i.e., compressing the abutment). The red dashed trace indicates the displacement of the stirrer, while the blue trace indicates the response of the photodiode sensor; This sensor response value shown in the blue trace was obtained by calculating Z = P Q R S, and is expressed in volts.

In the following claims and in the preceding description of the invention, except where the context otherwise requires due to express language or necessary implication, the word "comprises" or variations such as "comprises" or "comprising" is used in a inclusive sense; that is, to specify the presence of the indicated characteristics but not to exclude the presence or addition of other characteristics in various embodiments of the invention.

Claims (15)

1. A system for evaluating grip safety, the system comprising: a contact surface (10) comprising a plurality of protuberances (16) extending from a base surface (18), wherein the contact surface (10) has at least a first contact surface region (12 ) and a second contact surface region (14), the first contact surface region being configured to resist slipping less than the second contact surface region, in which the protuberances (16) of the first surface region contact surfaces (12) extend outside the base surface a distance less than the protuberances of the second contact surface region (14) and The system further comprises a sensor (33) to detect slip in the first region of the contact surface. 2. System according to claim 1, wherein the protuberances are deformable. 3. System according to claims 1 or 2, wherein the protuberances are configured to move independently of each other. 4. A system as claimed in any of the preceding claims, wherein the uncompressed distance from the base surface of the protuberances in the first contact region is less than that of the protuberances in the second contact region. 5. A system as claimed in any of the preceding claims, wherein the protrusions are configured such that in use under a constant gripping pressure the normal force on a protrusion in the first contact region is less than that of a protuberance in the second contact region. 6. A system as claimed in any of the preceding claims, wherein each protuberance comprises an internal cavity extending from the base surface towards a distal end of the protuberance. 7. A system according to claim 6, wherein a sensor is located behind the base surface at a location adjacent to each cavity. 8. A system as claimed in claim 7, wherein an opening having a diameter smaller than the cavity is located between the sensor and the cavity. 9. A system as claimed in claim 8, wherein the sensor comprises a quadrant photodiode configured to detect light emanating from LEDs located on one side of the cavity of the base surface within a given protuberance, which It is reflected from a reflector located at or near the distal end of the cavity and traveling through the opening to the sensor. 10. A system as claimed in any of the preceding claims, wherein the protuberances each have an internal cavity, and wherein the sensor comprises a CCD array, the system being configured such that the CCD array detects the light emanating from the base surface into the cavity, reflecting from a reflector located at the distal end of the cavity, and traveling through an opening in the base surface toward the sensor. 11. A system as claimed in any of the preceding claims, wherein the protuberances each have an internal cavity, and wherein the sensor comprises a light-sensitive CMOS matrix, the system being configured such that the Light-sensitive CMOS array detects light emanating from the base surface into the cavity, reflecting from a reflector located at the distal end of the cavity, and traveling through an opening in the base surface toward the sensor. 12. A method for evaluating grip security, the method comprising detecting slippage on a contact surface (10), wherein the contact surface comprises a plurality of protuberances (16) extending from a base surface (18). ), wherein the contact surface (10) has at least a first contact surface region (12) and a second contact surface region (14), the first contact surface region being configured to resist slippage less that the second contact surface region, the protuberances (16) in the first contact region extend outside the base surface a smaller distance than the protuberances in the second contact region, where the method comprises the step of detecting sliding in the first contact surface region by means of a sensor, preferably wherein the protrusions are positioned to form an array of protrusions at the base and the movement of the individual protrusions is independent or partially independent of each other. 13. A method as claimed in claim 12, wherein each protuberance is associated with a sensor configured to detect light emanating from the base surface into the cavity, reflecting from a reflector located at the distal end of the cavity. , and traveling through an opening in the base surface toward the sensor. 14. A method according to claim 12, wherein the plurality of protuberances extend from the base surface to a tip, and the method comprises measuring the displacement of the tip in three dimensions, and estimating the forces applied to the tip in three dimensions at the moment of sliding. 15. A method according to claim 12, wherein the plurality of protuberances extend along an axis from the base surface to a tip, and the method comprises measuring the displacement of the tip in three dimensions with a high resolution so that tip vibration is measured, and use the vibration measurement to estimate texture or slip.